Transistor switching circuits



July 21, 1964 .1. w. STREATER 3, I A TRANSISTOR SWITCHING CIRCUITS v 7 Filed Nov. 22, 1960 IO D-C PULSE GENERATOR TO ADDITIONAL CHAN NELS IO A D-C PULSE TO GENERATO ADDITIONAL CHANNELS D-C PULSE B R TO GE E ADDITIONAL CHANNELS I i R IN I 0/ ls INVENTOR. JACK w. STREATER ATTORNEY United States Patent O 3,142,065 TRANSISTOR SWITCHING CIRCUITS Jack W. Streater, Lyndhnrst, Ohio, assignor to Clevite Corporation, a corporation of Ohio Filed Nov. 22, 1960, Ser. No. 71,054 19 Claims. (Cl. 346- 74) This invention relates to switching circuits generally and, in particular, to switching circuits in which a signal of relatively low potential is used to control, and specifically to initiate or terminate flow of current from, a

on the record medium.

unit as, for example, the sticking of a relay in one position or another may cause an erroneous record and, consequently, it is desirable in some critical application, that the apparatus be fail-safe, i.e., provide a positive check against malfunction.

It will be understood that a relatively high voltage is required across the writing electrodes to make a satisfactory trace on the recording medium and it is necessary to exert a positive switching control over the writing voltage correlated to the occurrence and cessation of the event being monitored so that the voltage across the electrodes is either on or off, i.e., respectively sufficient or insufiicient to make a trace. Heretofore, this has been accomplished by use of a continuous D.-C. voltage controlled by mechanical switching devices, subject to all their usual inherent disadvantages such as contact deterioration, inertia of the moving element, etc., as well as the wear and other timeand service-connected deterioration inevitably incident to mechanical devices of any sort.

While the shortcomings of mechanical switching have .been successfully averted in many fields by resort to .solid state electronic switching devices such as transistors and diodes, the application of such devices has heretofore been limited by their relatively low voltage-handling capacity.

With this state of the art in view, it is the fundamental object of the present invention to provide novel switch- ,ing circuits overcoming or mitigating at least one of the problems outlined above.

A more specific object is the provision of improved electronic switching systems having no moving parts and capable of controlling relatively high potentials.

t Another object is the provision of novel electric-writing 3,142,065 Patented July 21, 1964 systems wherein switching between writing and not- Writing conditions is accomplished by solid state switching devices. j

A further object is the provision of an improved electric writing system wherein the record trace is made on an electrosensitive medium by the application of a pulsed D.-C. potential.

A still further object is the provision of an improved electric-writing events recorder wherein a trace is recorded on an electrosen sitive recording medium during the occurrence or non-occurrence of an event and a separate trace is made during the converse condition, i.e., nonoccurrence or occurrence, respectively, of the event, as proof that the existence or non-existence of one trace on the record is due to its correlation to the event being monitored and not to malfunctioning of the recording unit.

These and additional objects of the invention are accomplished by an electric-writing system according to the present invention which comprises, in combination, a

writing stylus electrode, means for generating and supplying to the stylus electrode a pulsating D.-C. potential, and, connected to the pulse-generating means in parallel with the stylus electrode, a conductive pathhaving inductive reactance selectively adjustable between a relatively high and a relatively low value.

In accordance with another of its features the invention contemplates switching circuits comprising a transistor having its emitter, collector and base electrode connected to form input and output circuits; a source of pulsating direct current at a potential exceeding the maximum operating voltage of the transistor; and a conductive path for the pulsating current having two parallelbranches one of which has a substantially fixed impedance of appreciable magnitude. Transformer means are provided having one winding of relatively low inductance connected across the output circuit of the transistor and a second winding of relatively higher inductance connected in series in the other branch of the conductive path. Consequently, application of a signal potential to the input circuit of the transistor such as to bias it to conduction causes a drop in the inductive reactance of the high inductance winding which then diverts current from the fixed impedance path. 7

Additional objects of the invention, its advantages, scope and the manner in which it may be practiced will be readily apparent to persons skilled in the art from the following description of specific exemplary embodiments read in conjunction with the subjoined claims and the annexed drawings in which like characters of reference generatorltl would vary with the particular application,

1 common emitter.

snsaoes one practical form of multi-channel electric writing system utilizes a generator capable of producing a peak output pulse of approximately 500 volts, a pulse width of about 100 microseconds and a pulse repetition frequency variable between one and 1000 per second.

The output of pulse generator is supplied via a bus 12, series resistance R and conductor 14 to a writing stylus electrode indicated symbolically by arrow 16. As shown in the drawings, stylus electrode 16 has its writing or discharge point directed at and disposed in close proximity to a conductive plate 18 connected to ground potential. Plate 18 serves as a counter-electrode, coacting with stylus electrode 16 to define a dielectric gap which is occupied in service by a suitable electrosensitive recording medium such as Teledeltos paper indicated by scroll 20. The record medium or chart is mounted on suitable feed and takeup rolls, not shown, constructed and arranged in any conveniently known manner to transport the chart at a selected speed. Plate or counter-electrode 18 serves as a support member for the chart passing under stylus electrode 16.

From the structure thus far described it will be seen that bus 12, series resistor R conductor 14, stylus electrode 16, and support plate 18 form a conductive path between pulse generator 10 and ground potential.

Connected in parallel with stylus electrode 16, between pulse generator 10 and ground, is one winding, the higher inductance winding W a transformer T, which has a second winding W of lower inductance. Transformer winding W is directly connected to stylus electrode 16 and, through resistor R to pulse generator 10. The other end of winding W is connected to ground potential by virtue of a conductor 22 connecting it to grounded support plate 18.

A transistor 24 having an emitter, collector and base, is connected to form input and output circuits having, in the illustrated embodiment, the emitter in common. Specifically, the output circuit of transistor 24 includes the collector and emitter which are connected, by conductors 26, 28 to respective ends of transformer winding The base electrode of transistor 24 is connected through a series resistor R to one of a pair of input terminals 30, 32 the other of which is connected to the The particular value of R depends on the gain at transistor 24 and the desired range of input signal potentials.

In the illustrated embodiment transistor 24 is of the NPN type although this is by no means essential. It is controlling only as to the polarity of the input signal potential applied to terminals 30, 32. It is pointed out that in this particular embodiment no bias is required.

For use in the particular writing system being described transformer T is conveniently of the ferrite core type and has a turns ratio (W :W of 12:1. The characteristics of pulse generator 10 affect the selection of the transformer. With the specific pulse generator output described above by way of example (viz., pulse amplitude, 500 volts; pulse width 100 microseconds; repetition frequency 1 to 1000 pulses per second) suitable parameters of the transformer for the particular application and a specific practical embodiment are:

Inductance of winding W At least 6 henrys Resistance of winding W Less than 1200 ohms Resistance of winding W Less than 25 ohms transistor 24 is of the type commercially designated as 2N35 and the values of R and R are 22,000 ohms and 1,000, respectively, the latter (R for an input signal range of 7-100 volts.

In service as an events monitor, suitable means (not shown) are provided for supplying across terminals 30, 32 a signal potential in response to the occurrence and/ or during the continuance of an event or condition being monitored or upon its cessation and/or continued nonoccurrence. For example, if a physical movement is to be monitored, a potential source and a switch actuated by the movement may be provided and arranged so that closure of the switch completes a circuit supplying the potential to terminals 30, 32. Of course, the monitoring of an electrical or electronic event can be accomplished by providing suitable means for developing a potential of suitable magnitude and polarity, correlated to the event. Thus current flow in a conductor can be monitored by inserting the conductor in series with the input terminals 30, 32.

The magnitude of the signal potential depends on the value of resistor R and the bias voltage required to cause conduction in the output circuit of transistor 24 as hereafter explained. The minimum value of the input signal potential would be that required to cause conduction of transistor 24 with resistor R removed; a maximum value would be imposed by the heating of resistor R The polarity of the input potential depends on transistor 24, the NPN transistor shown requiring a positive signal whereas a PNP transistor would require a negative signal. In the illustrated embodiment with pulse generator 10 in operation stylus electrode 16 writes, i.e., makes a trace on the record medium 20, in the absence of a signal across input terminals 30, 32. Under these circumstances, the

transistor is non-conductive so that there is effectively an open circuit in low inductance winding W of transformer T; consequently, due to the inductive coupling between windings W and W the inductive reactance of winding W is a maximum. Transformer T is designed so that the inductive impedance of winding W under these conditions, i.e., with W open-.circuited, is sufiiciently in excess of the impedance presented by the electrosensitive medium 20 between the stylus electrode 16 and plate 18 that the primary current flow is diverted to the stylus electrode and is suflicient to form a visible trace on the record medium in passing to ground.

When a signal potential of proper polarity and magnitude is applied to input terminals 30, 32 transistor 24 is biased to conduction so that transformer winding W is, in effect, substantially short-circuited, reducing the inductive reactance of Winding W sufficiently to shunt a major proportion of the pulse current from the stylus electrode causing it to stop writing.

Thus it will be seen that transistor 24, in conjunction with transformer T, functions as a variable impedance device, the impedance being selectively adjustable between respective conditions of high and low value. By virtue of the fact that the writing potential is applied in pulses rather than continuously as in prior art devices, the transformation achieved by the turns ratio of the transformer becomes possible with the ultimate result that the transistor is made capable of controlling much larger voltages its emitter common to both its input and output circuits. Thus, the emitters of both transistors 24, 36, are interconnected to one input terminal, e.g., 32. The other input terminal, 30', is connected to the base of transistor 34 through a series resistor R The collector of tran- .sistor 34 is coupled to the base of transistor 24 through a series resistor R A fixed bias potential from a source represented by battery B is applied through a dropping resistor R to a point between the collector of transistor 34 and resistor R The magnitude of the bias potential and its polarity are such as normally to maintain transistor 24 in a conducting condition. Inasmuch as the illustrated embodiment employs NPN transistors, the bias potential is of positive polarity.

From FIGURE 2 it will be seen that the bias potential represented by battery B is effectively in parallel with the output (emitter-collector) circuit of transistor 34 and the input (emitter-base) circuit, including R of transistor 24. The magnitude of the bias and the value of R and R are selected, with reference to each other and the output impedance of transistor 34 and the input impedance of transistor 24 that, with transistor 34 cutoff, transistor 24 is biased to conduction and with transistor 34 conducting sufiicient current is diverted thereby that the potential across the input of transistor 24 drops below the value required to maintain its conduction and this transistor is, therefore, cut off. This arrangement enables the following operation of the writing system.

In the absence of a signal voltage across input terminals 30, 32', transistor 34 is non-conductive while the bias potential applied across the input of transistor 24 causes it to conduct; as a result transformer winding W is effectively short-circuited causing the inductive reactance in winding W to fall to its low value shunting current around the Writing gap between electrode 16 and plate 18 so that no trace is made on the record chart (not shown in FIG- URE 2).

When a signal of proper polarity and amplitude is applied to input terminals 38', 32, transistor 34 becomes conductive shunting current around the input circuit of transistor 24 by providing a low impedance path for the bias voltage applied by battery B. Consequently, transistor 24 is rendered non-conductive resulting in a high impedance in its output circuit and, therefore, in winding W of transformer T, so that writing current is diverted to Transformer T and Pulse generator 10: As described for FIGURE 1.

Transistors 24, 34: 2N35 Battery B: +10 v.

With these circuit constants the system operates on input signal in the range of from about 7 to 100 volts. For lower voltages the value of R can be reduced.

Another embodiment of the invention, illustrated in FIGURE 3 contemplates a writing system having a circuit in which writing is initiated by the application of a signal and a circuit in which writing has terminated by the application of a signal, in order to provide a failsafe arrangement in which one or the other of a pair of stylus electrodes is recording a trace at all times, i.e., so that the initiation and termination of the event being monitored has the effect of switching the trace from one stylus electrode to the other.

In this way the absence of a trace by either stylus of the pair, of a simultaneous trace by both stylii indicates malfunction of the monitoring system.

Referring to FIGURE 3, the fail-safe circuit, briefly stated, is composed of a parallel combination of the respective circuits shown in and described in conjunction with FIGURES 1 and 2. Thus the system comprises a pair of stylus electrodes 16, 16' individually connected through respective series resistors R R' via bus 12 to a D.-C. pulse generator 10. Each stylus electrode is dis- '6 posed opposite a counter-electrode and chart support member, which for clarity of illustration is shown in the drawings as two separate members 18, 18 but which in a practical application usually would be a unitary element.

A' respective't'ransformer T andT' is provided for each stylus electrode, 16 and 16'. The high inductance windings W W are connected in parallel with respective stylus electrodes 16, 16'. I 'Respective transistors 24, 24' each connected with its emitter common to its input and output circuits, are provided for each stylus electrode-transformer assembly T, 16 and T, 16'. The respective low inductance windings W W of each transformer T, T' are connected across the'output (emitter-collector) circuitsof respective transistors 24, 24'. i i

A third transistor 34 is provided, preceding in cascade, one of the other two, in the illustrated embodiment, transistor 24'. The emitters of each of the three transistors 24, 24', 34 are electrically interconnected to' and constitute one signal input terminal -32. The bases of transistors 24 and 34 are connected through respective series resistors R and R to the other input terminal 30. The base of transistor 24' is connected through a series resistor R to the collector of transistor 34. Abias potential supplied by battery B, is connected across output circuit of transistor 34 and the input circuit of transistor 24 in the manner described with respect to FIGURE 2.

The operation of the circuit illustrated in FIGURE 3 is as follows: In the absence of a signal applied to input terminals 30, 32', transistor 24 is non-conductive and, in the manner described in conjunction with theFIGURE 1 circuit, the associated stylus, 16, records a trace; The

absence of a signal at the input terminals also renders transistor 34 non-conductive with the result that the bias 24 which, as previously described, causes stylus'16 to start writing. Thus, it will be seen that in the absence of a signal stylus electrode 16 leaves a continuous trace on the record chart while notr -ace is madeby the stylus electrode 16'; on the application of a signal, stylus electrode 16' starts and stylus electrode 16, simultaneously stops writing.

' Any finite number of pairs of electrodes and associated transformers and transistors can be employed to provide plural channels each having respective input terminals and recording on a common or separate record charts.

While there have been described what at present are believed to be the preferred embodiments of this invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is aimed,

therefore, to cover in the appended claims all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed and desired to be secured by United States Letters Patent is:

1. An electric writing system comprising, in combination: a Writing stylus electrode; means for generating and supplying to said stylus electrode a pulsating DC.

'potential; a pair of transistors each having an emitter,

collector and base; means coupling said transistors in cascade with their emitters in common; transformer means having a first winding connected between the emitter and collector of one of said transistors and having a second winding of relatively higher inductance inductively coupled to said first winding and connected to said pulse generating means in parallel with said stylus electrode; and means for applying a potential across said coupling means whereby said one transistor is biased to conduction when the other transistor is non-conductive.

2. An electric writing system comprising, in combination: at least two writing stylus electrodes; means for generating and supplying to said stylus electrodes a pulsating D.-C. potential; a transistor for each of said stylus electrodes each having a respective input and output circuit; transformer means for each of said stylus electrodes each having a first winding connected across the respective output circuit of one of said transistors and a second winding, of higher inductance than, and inductively coupled to, the first winding, connected to said pulse generating means in parallel with a respective one of said stylus electrodes; an additional transistor preceding, in cascade, a first of said other transistors; means for applying in parallel with the output of said additional transistor and the input of said first transistor a potential normally effective to render said one first transistor conductive when said additional transistor is non-conductive; and means connecting the respective inputs of said additional transistor and a second of said other transistors in parallel.

3. An electric writing system comprising, in combination: at least two writing stylus electrodes; means for generating and supplying to said stylus electrodes a pulsating D.-C. potential; a transistor for each of said stylus electrodes each transistor having an emitter, a collector and a base electrode; transformer means for each of said stylus electrodes each having a low impedance winding connected between the emitter and collector electrodes of a respective one of said transistors and, inductively coupled to the low impedance winding, a second winding of relatively higher impedance connected to said pulse generating means in parallel with a respective one of said stylus electrodes; an additional transistor having an emitter, a collector and a base electrode, the respective emitters of all said transistors being interconnected to constitute one signal input terminal; means interconnecting the base electrode of said additional transistor and a first of the other transistors to constitute a second input terminal; means including a series resistive impedance connecting the collector of said additional transistor to the base of a second of the other transistor; and means for applying to a point between said resistive impedance and the collector of said additional transistor, a potential with respect to said emitters effective to render said first other transistor conductive when the additional transistor is non-conductive.

4. An electric writing system comprising, in combination: at least one writing stylus electrode having a writing point; a conductive support member at a ground potential disposed in close proximity to the point of said stylus electrode and adapted to establish with said point a dielectric gap occupied in service by a transient electrosensitive record medium; means for generating D.-C. voltage pulses having a peak amplitude in the order of 500 volts and the pulse width in the order of about 100 microseconds; means connecting said stylus to said pulse generating means; transformer means having a primary winding and, inductively coupled thereto, a secondary winding, said primary winding having an inductance of at least about 6 henrys and a resistance of less than about 1200 ohms, said secondary winding having a resistance of less than about 25 ohms, said transformer having a turns ratio of about 12 to 1 and being characterized in that the sum of the primary leakage inductance and the product of the turns ratio squared and secondary leakage inductance is less than 100 millihenrys; means connecting said secondary winding between said stylus electrode and supporting member; a transistor having an emitter, a collector, and a base electrode, the emitter and collector of said transistor being connected to respective ends of the primary winding of said transformer, the emitter and base electrode constituting input terminals for said transistor.

5. An electric writing system according to claim 4 including a second transistor substantially identical to the first said transistor, said first and second transistors having their respective emitters interconnected to one of said input terminals, the base of said second transistor constituting the other input terminal; means including a resistive impedance interconnecting the base of said transistor to the collector of said second transistor; means for applying to a point between said resistive impedance and the collector of said second transistor a potential with respect to said emitters effective, in the absence of a signal across said input terminals, to bias said first transistor to conduction and said second transistor to cut-01f.

6. An electric writing system according to claim 5 including an additional stylus electrode connected to said pulse generating means and having a writing point disposed in close proximity to and defining with said support member, a second dielectric gap; a second transformer substantially identical to that first mentioned and having one end of its secondary winding connected to said additional stylus electrode and its other end to said conductive support member; a third transistor substantially identical to said first and second transistors and having its collector and emitter connected to respective ends of the primary winding of said second transformer, its emitter to the emitters of said first and second transistors, and its base electrode coupled to said second input terminals.

7. An electric writing system comprising, in combination: a Writing stylus electrode for producing a record in response to voltage and current conditions of predetermined magnitude; a source of pulsating D.-C. potential connected to said stylus electrode to apply a pulsating D.-C. potential thereto; a transistor having an input and an output circuit; transformer means having a first winding connected across the output circuit of said transistor and a second winding of higher inductance than said first winding inductively coupled to said first winding and connected to said pulse generating means in parallel with said stylus electrode whereby the effective circuit impedance of said second winding determines the voltage and current conditions of said stylus electrode; the pulsating current flow through said second winding and the inductive coupling of said windings resulting in impedance reflection between said windings; and means for selectively applying an input signal to the input circuit of said transistor to effect conditions of low and high impedance in its output circuit to thereby reflect low and high impedances respectively across said second winding to control writing operation of said stylus electrode.

8. An electric writing system according to claim 7, wherein said signal applying means includes a second transistor preceding in cascade said one transistor and having conductive and non-conductive states; and means for applying a bias potential in parallel with the output of said second, and the input of said one transistor whereby said one transistor is biased to said low impedance condition when said second transistor is conductive.

9. An electric writing system comprising, in combination: a writing stylus electrode; means for generating and supplying to said stylus electrode a pulsating D.-C., potential; a transistor having emitter, collector and base electrodes and having a high current impedance when non-conductive and a low circuit impedance when conductive between its emitter and collector electrodes; transformer means having a first winding connected between the emitter and collector of said transistor and, a second winding of relatively higher inductance inductively coupled to: said first Winding and connected to saidpulse generating means in parallel with said stylus electrode; the flow of pulsating current through said second winding and the inductive coupling of said windings resulting in impedance reflection between said windings; and means for selectively applying between the emitter and base of said transistor a signal potential effective to selectively render the transistor conductive and non-conductive in its emitter-collector circuit to thereby selectively reflect low and high impedances 9 across said second Winding to control writing operation of said stylus electrode.

10. Inan electric writing system utilizing an electrosensitive recording medium, the combination comprising: a stylus electrode cooperative with a counter electrode for producing a trace on the recording medium in re- Sponseto voltage and current conditions across said electrodes of predetermined magnitude; a source of pulsating D.-C. potential for said electrodes; a transformer having one winding of relatively low inductance and a second inductively coupled winding of relatively higher inductance; means for electrically connecting said second winding in circuit with said source and electrodes whereby the voltage and current conditions across said electrodes are dependent on the effective circuit impedance of said second winding; the pulsating current flow through said second winding and the inductive coupling of smd windings resulting in impedance reflection between said windings; and variable impedance means coupled to said one winding for producing a reflected impedance across said second winding variable between first and second magnitudes for varying the voltage and current conditions across said electrodes to thereby control Writing operation of said electrodes.

11. In an electric writing system utilizing an electrosensitive recording medium, the combination comprising: a stylus electrode cooperative with a counter electrode for producing a trace on the recording medium in response to voltage and current conditions across said electrodes of predetermined magnitude; a source of pulsating D.-C. potential for said electrodes; a transformer having one winding of relatively low inductance and a second inductively coupled winding of relatively higher inductance; means for electrically connecting said second winding in circuit with said source and electrodes whereby the voltage and current conditions across said electrodes are dependent on the effective circuit impedance of said second Winding; the pulsating current flow through said second winding and the inductive coupling of said windings resulting in impedance reflection between said windings; and a transistor having an input and output circuit and a variable output impedance dependent upon the state of conduction of said transistor in response to the application of an input signal to said input circuit; said output circuit being coupled to said one winding to reflect said variable output impedance across said second winding to vary the voltage and current conditions across said electrodes to thereby control writing operation of said electrodes in response to variations in the input signal.

12. An electric writing system as claimed in claim 11 wherein a second transistor having input and output circuits has its output circuit coupled to the input circuit of the first said transistor; and means are provided for applying a potential in parallel with the output of said second transistor and the input of the first said transistor whereby the first said transistor is biased conductive to a low impedance condition when said second transistor is nonconductive.

13. An electric writing system as claimed in claim 12 wherein said transistors have base, emitter and collector electrodes and are coupled in cascade with their emitters in common, the input circuits of said transistors being connected to the emitter and base electrodes of said transistors respectively and the output circuits of said transistors being connected to the collector and emitter electrodes, respectively.

14. In an electric writing system utilizing an electrosensitive recording medium, the combination comprising: a stylus electrode cooperative with a counter electrode for producing a trace on the recording medium in response to voltage and current conditions across said electrodes of predetermined magnitude; a source of pulsating D.-C. potential for said electrodes; a transformer having one winding of relatively low inductance and a second inductively coupled winding of relatively higher inductance;

means for electrically connecting said second winding in circuit with said source and electrodes to establish a pulsating current flow through said second winding and voltage and circuit conditions across said electrodes dependent on the eflective circuit impedance of said second Winding; the pulsating current flow through said second winding and the inductive coupling of said windings resulting in impedance reflection between said windings; a transistor having emitter, collector and base electrodes and having a variable circuit impedance between said emitter and collector electrodes dependent upon the state of conduction of said transistor in response to the application of an input signal across said base and emitter electrodes; said emitter and collector electrodes being coupled to said one Winding to reflect said variable circuit impedance of said transistor across said second winding to vary the voltage and current conditions across said electrodes in response to variations in conductivity of said transistor to thereby control writing operation of said electrodes.

15. In an electric writing system having a source of pulsating direct current, the combination comprising an electrically conductive circuit having two parallel circuit branches adapted to be connected to the direct current source; electrically operative recording means responsive to a pulsating direct current connected in one of said circuit branches; a transformer having one winding of relatively low inductance and a second inductively coupled winding of relatively higher inductance; said second winding being electrically connected in the other of said circuit branches to define a variable inductive impedance; the pulsating current flow through said second winding and the inductive coupling of said windings resulting in impedance reflection between said windings; and variable impedance means coupled to said one winding to reflect a variable impedance across said second winding to vary the relative magnitudes of current flow in said circuit branches to thereby control operation of said recording means.

16. In an electric writing system utilizing an etlectrosensitive medium, the combination comprising: a stylus electrode cooperative with a counter electrode for producing a trace on the recording medium in response to voltage and current conditions across said electrodes of predetermined magnitude; a source of pulsating DC. potential connected to said electrodes; a transformer having one winding of relatively low inductance and a second inductively coupled winding of relatively higher inductance; circuit means connecting said second winding to said source in parallel with said electrodes to establish a pulsating current flow through said second winding and to divert current flow from said electrodes in accordance with the impedance of said second winding to thereby vary the voltage and current conditions across said electrodes; the pulsating current flow through said second winding and the inductive coupling of said windings resulting in impedance reflection between said windings; and variable impedance means having first and second impedance conditions coupled to said one winding for producing a reflected impedance across said second winding variable between first and second magnitudes for varying the voltage and current conditions across said electrodes to thereby control writing operation of said electrodes.

17. An electric writing system as claimed in claim 16 wherein. said variable impedance means comprises a transistor having input and output circuits and a variable output impedance dependent upon the state of conduction of said transistor in response to an input signal, said output circuit being coupled to said second winding.

18. An electric writing system as claimed in claim 17 wherein a second transistor having input and output circuits has its output circuit coupled to the input circuit of the first said transistor; and means are provided for applying a potential in parallel with the output of said second transistor and the input of the first said transistor whereby the first said transistor is biased conductive to a low im- 1 1 pedance condition when said second transistor is nonconductive.

19. An electric writing system as claimed in claim 18 wherein said transistors have base emitter and collector electrodes and are coupled in cascade with their emitters in common, the input circuits of said transistors being connected to the emitter and base electrodes of said transistors, respectively, and the output circuits of said transistors being connected to the collector and emitter electrodes of said transistors, respectively.

References Cited in the file of this patent UNITED STATES PATENTS Hester et a1. Mar. 6, Gauld Nov. 27, Russell Jan. 26, Bulleyment May 24, Farber June 6, Burski Aug. 1, 

2. AN ELECTRIC WRITING SYSTEM COMPRISING, IN COMBINATION: AT LEAST TWO WRITING STYLUS ELECTRODES; MEANS FOR GENERATING AND SUPPLYING TO SAID STYLUS ELECTRODES A PULSATING D.-C. POTENTIAL; A TRANSISTOR FOR EACH OF SAID STYLUS ELECTRODES EACH HAVING A RESPECTIVE INPUT AND OUTPUT CIRCUIT; TRANSFORMER MEANS FOR EACH OF SAID STYLUS ELECTRODES EACH HAVING A FIRST WINDING CONNECTED ACROSS THE RESPECTIVE OUTPUT CIRCUIT OF ONE OF SAID TRANSISTORS AND A SECOND WINDING, OF HIGHER INDUCTANCE THAN, AND INDUCTIVELY COUPLED TO, THE FIRST WINDING, CONNECTED TO SAID PULSE GENERATING MEANS IN PARALLEL WITH A RESPECTIVE ONE OF SAID STYLUS ELECTRODES; AN ADDITIONAL TRANSISTOR PRECEDING, IN CASCADE, A FIRST OF SAID OTHER TRANSISTORS; MEANS FOR APPLYING IN PARALLEL WITH THE OUTPUT OF SAID ADDITIONAL TRANSISTOR AND THE INPUT OF SAID FIRST TRANSISTOR A POTENTIAL NORMALLY EFFECTIVE TO RENDER SAID ONE FIRST TRANSISTOR CONDUCTIVE WHEN SAID ADDITIONAL TRANSISTOR IS NON-CONDUCTIVE; AND MEANS CONNECTING THE RESPECTIVE INPUTS OF SAID ADDITIONAL TRAN SISTOR AND A SECOND OF SAID OTHER TRANSISTORS IN PARALLEL. 